Effects of direct optical ablation and sequent thermal ablation on an ultrafast pulsed laser microprocessing

Abstract

An ultrafast pulsed laser processing for materials can obtain submicometer- to nanometer-sized parts or patterns because the heat cannot diffuse in time during the ultrafast pulsed duration and the multiphoton absorption leads to a threshold of ablation. An ultra-fast pulsed laser is first absorbed due to optical penetration. Therefore, the direct optical penetration or coulomb explosion leads to the initial optical ablation during ultrafast pulsed duration without thermal diffusion. After the direct optical ablation and sequent thermal diffusion, the thermal ablation occurs for the residual heat of an ultrafast pulsed laser maintaining the material temperature above the evaporated temperature. This study uses Laplace transform method to show the effects of optical ablation and thermal ablation, respectively. The results reveal that both optical and thermal ablations follow Beer's exponential law as experimental observations in the published papers. The optical ablation triggered by the only energy source of directly incident ultrafast pulsed laser governs the main ablation depth, and then thermal ablation induced by the residual heat after the optical ablation gives the enhancement of the ablation depth. The optical ablation is preferred for obtaining the better processing quality of an ultrafast pulsed laser due to no thermal damage of thermal diffusion-induced ablation. The ablated depth per pulse versus laser fluence predicted by this work agrees with that measured by the published paper. This study will provide the closed-form solution to elucidate direct optical ablation and sequent thermal ablation for the ultra-fast pulsed laser photo-thermal processing.